In modern society, the trend among consumers to prefer fresh products leads to that chilled food is more and more competing with frozen and ambient counterparts. Fresh or chilled food is more sensitive, thus enhancing the demand from food consumers to be able to trust that the food they buy and consume is safe. One important issue in the transfer from ambient or frozen food products to chilled food is therefore the issue of safety and quality which is identified as an increasing concern, emphasized frequently in industry and media.
The demand for fresh chilled food has created a need for distribution of fresh products around the year. Fresh and chilled foods require both quicker distribution due to shorter shelf life, and better product and package integrity due to safety and quality issues at the point of consumption. The two most important factors concerning the shelf life are time and temperature. Most of the deteriorating changes that take place in food are temperature dependent and occur at a slower rate at lower temperatures. In addition to the time and temperature parameters, the natural presence of oxygen in food products and packaging environments, hastens chemical breakdown and microbiological spoilage of food. Traditional packaging methods are therefore largely being replaced by newer techniques where other gases than oxygen are filled in the package headspace. Modified Atmosphere Packaging (MAP) is one example of such techniques, used to reduce the oxygen content and to prolong the shelf life of the product it contains.
Due to food deteriorating aspects, the food handling, preservation and packaging have become important aspects of great public interest and concern. The MAP technology theoretically meets the new consumer requirements of prolonged shelf-life for chilled or fresh products, and the MAP packaging concept is growing in importance in the food packaging market. The main benefit of MAP is argued to be the reduction of indirect costs due to less product waste. But with the increased quality and safety concerns, the importance to be able to assess the status of packed food to ensure its quality and suitability for consumption must also be stressed. Many sensing techniques have been developed; however, most of them destroy the packaged sample and thereby cause waste of both products and packages. E.g., gas chromatography and other sampling techniques require puncturing the package for gas extraction, while measurement techniques should preferably be non-intrusive in nature, in order to maintain packaging integrity and reduce waste of samples.
Modified Atmosphere Packaging
Oxygen is a very reactive gas, naturally present in air. Most packaged food items come with a natural headspace of air between the product and the package. Oxygen is thus frequently present in the headspace. Due to the high activity of this molecule, it is important to control its concentration in order to secure quality and increase shelf-life of the product. Generally, it is desirable to reduce its concentration from ambient (21 percent) to a few percent or to a completely oxygen-free environment. By replacing the oxygen in the headspace the oxidative processes are reduced and the shelf life of the food product is extended.
In order to reduce the natural concentration of oxygen in air, food products are packaged in modified or controlled atmosphere. Modified atmosphere packaging (MAP), as used in this disclosure, is defined by Hintlian and Hotchkiss as “the packaging of perishable product in an atmosphere which has been modified so that its composition is other than air”. In MAP the natural oxygen content inside the package is replaced with other gases such as carbon dioxide (CO2) or nitrogen (N2). Frequently, the gas composition is actively changed at the time of packaging, either by flowing gas during the packaging or by first subjecting the product to vacuum followed by inlet of the desired gas mixture.
Fresh products such as fruits or cured meat are the most common products to pack in modified atmosphere. But more frequently, products such as high quality fruit juices are packed in modified atmosphere in order to reduce deterioration from oxidation and to extend the product shelf-life. The modified atmosphere is slowing chemical and biochemical deteriorative processes as well as slowing or preventing the growth of spoilage organisms. For high quality fruit juices, nitrogen is the most common gas to use in the headspace for replacement of oxygen. Nitrogen is an inert gas and does not dissolve well in water or lipids and thus ensures that a package looks filled and is not collapsing. It has no anti-microbial activity but by displacing oxygen in the headspace of packages the oxidative processes of the products are delayed.
For MAP, the package integrity and tightness are necessary to maintain the correct composition of gas inside the package, thus securing that no gas exchange takes place between the package and its environment. It is important to control the oxygen permeability in order to secure the prolonged shelf life of liquid food products, such as for example high quality orange juice. Packages with a modified atmosphere therefore require packaging materials that are tight to gas transfers, in order to avoid oxygen from getting into the headspace. For packages that have an exchange, (intended or not intended) with the outside environment, equilibrium will be reached between the inside and outside of the package over time with oxygen entering the headspace. Furthermore, in a modified atmosphere package, equilibrium of gas concentrations inside the package may arise due to the interaction between the product and the gas contained in the package. A measurement method or apparatus is desired for assessing the sealing tightness of the packaging. Such method or apparatus is for instance suitable to detect the headspace gas composition that will include the simultaneous gas movement from permeation and through the exchange from product to headspace, in a dynamic MAP system.
Non-Intrusive Gas Sensing of Food Packages
Measurement of oxygen contents in sealed packages might be the most pertinent aspect of monitoring the gas composition in the headspace of packages. Oxygen assessment can be made by performing optical measurements, using small sensor disks prepared to change in colour in the presence of oxygen. Alternatively and more commonly used, the sensor disk is prepared with a ruthenium- or platinum-containing dye, with fluorescence properties that decreases with the amount of oxygen. However, these techniques are intrusive from the point of view that the small disks have to be introduced in the package at the time of sealing. In addition there is a cost and a safety aspect ensuring that the active reactive agent does not influence the product or the consumer. Hence, an alternative non-intrusive method or apparatus would be advantageous.
From a safety and consumer perception aspect, non-intrusive measurements, without devices put on or inside the package are therefore called for. In addition all extra items incur an extra cost, in terms of direct costs and indirect costs such as extra handling time, machine investments etc. Non-intrusive measurements also allows for measurements over time for packages during its entire shelf life.